What I tend to do is use AirPlay 2 to connect to a single Sonos speaker, and use the Sonos software to then ‘group’ that single speaker with any number of Sonos speakers. I find the bandwidth necessary to use AirPlay 2 to send to multiple speakers, rather than just one, to be overwhelming, in particular to my iPhone or iPad. Since it’s a habit now, I have never tried to send to multiple AirPlay 2 targets using the Apple TV or a Mac.
I have never been able to send anything beyond stereo via AirPlay 2. What I do sometimes is to send video to my AppleTV, which then sends the video and Audio to my TV set, which passes the audio via HDMI ARC to my Sonos Arc. Easier that way.